TP13.3: Spring Cloud Gateway

🎯 Objectives of the Lab

• Create an API Gateway with Spring Cloud Gateway
• Configure routing to services
• Implement custom filters
• Configure rate limiting
• Add fallbacks for unavailable services

Estimated duration: 1h30

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📋 Architecture

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Part 1: Create the API Gateway (20 min)

1.1 Create the Project

pom.xml:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 
         https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>3.2.0</version>
    </parent>
    
    <groupId>com.example</groupId>
    <artifactId>api-gateway</artifactId>
    <version>1.0.0</version>
    
    <properties>
        <java.version>17</java.version>
        <spring-cloud.version>2023.0.0</spring-cloud.version>
    </properties>
    
    <dependencies>
        <!-- Spring Cloud Gateway -->
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-gateway</artifactId>
        </dependency>
        
        <!-- Eureka Client -->
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-netflix-eureka-client</artifactId>
        </dependency>
        
        <!-- Circuit Breaker -->
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-circuitbreaker-reactor-resilience4j</artifactId>
        </dependency>
        
        <!-- Actuator -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-actuator</artifactId>
        </dependency>
    </dependencies>
    
    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>org.springframework.cloud</groupId>
                <artifactId>spring-cloud-dependencies</artifactId>
                <version>${spring-cloud.version}</version>
                <type>pom</type>
                <scope>import</scope>
            </dependency>
        </dependencies>
    </dependencyManagement>
    
    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>
</project>

1.2 Main Application

package com.example.gateway;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class ApiGatewayApplication {

    public static void main(String[] args) {
        SpringApplication.run(ApiGatewayApplication.class, args);
    }
}

1.3 Route Configuration (YAML)

# src/main/resources/application.yml
server:
  port: 8080

spring:
  application:
    name: api-gateway
  cloud:
    gateway:
      routes:
        # Route User Service
        - id: user-service
          uri: lb://user-service
          predicates:
            - Path=/api/users/**
          filters:
            - name: CircuitBreaker
              args:
                name: userServiceCB
                fallbackUri: forward:/fallback/users
        
        # Route Product Service
        - id: product-service
          uri: lb://product-service
          predicates:
            - Path=/api/products/**
          filters:
            - name: CircuitBreaker
              args:
                name: productServiceCB
                fallbackUri: forward:/fallback/products
      
      # Automatic discovery (optional)
      discovery:
        locator:
          enabled: true
          lower-case-service-id: true

# Eureka
eureka:
  client:
    service-url:
      defaultZone: http://localhost:8761/eureka/

# Actuator
management:
  endpoints:
    web:
      exposure:
        include: health,info,gateway
  endpoint:
    gateway:
      enabled: true

# Resilience4j
resilience4j:
  circuitbreaker:
    instances:
      userServiceCB:
        slidingWindowSize: 10
        failureRateThreshold: 50
        waitDurationInOpenState: 10000
        permittedNumberOfCallsInHalfOpenState: 3
      productServiceCB:
        slidingWindowSize: 10
        failureRateThreshold: 50
        waitDurationInOpenState: 10000

# Logging
logging:
  level:
    org.springframework.cloud.gateway: DEBUG

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Part 2: Fallback Controller (15 min)

2.1 Create the Fallback Controller

package com.example.gateway.controller;

import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import java.time.LocalDateTime;
import java.util.Map;

@RestController
@RequestMapping("/fallback")
public class FallbackController {
    
    @GetMapping("/users")
    public ResponseEntity<Map<String, Object>> userServiceFallback() {
        return ResponseEntity
            .status(HttpStatus.SERVICE_UNAVAILABLE)
            .body(Map.of(
                "status", "SERVICE_UNAVAILABLE",
                "message", "User Service is currently unavailable. Please try again later.",
                "timestamp", LocalDateTime.now().toString(),
                "service", "user-service"
            ));
    }
    
    @GetMapping("/products")
    public ResponseEntity<Map<String, Object>> productServiceFallback() {
        return ResponseEntity
            .status(HttpStatus.SERVICE_UNAVAILABLE)
            .body(Map.of(
                "status", "SERVICE_UNAVAILABLE",
                "message", "Product Service is currently unavailable. Please try again later.",
                "timestamp", LocalDateTime.now().toString(),
                "service", "product-service"
            ));
    }
    
    @GetMapping("/default")
    public ResponseEntity<Map<String, Object>> defaultFallback() {
        return ResponseEntity
            .status(HttpStatus.SERVICE_UNAVAILABLE)
            .body(Map.of(
                "status", "SERVICE_UNAVAILABLE",
                "message", "Service temporarily unavailable",
                "timestamp", LocalDateTime.now().toString()
            ));
    }
}

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Part 3: Custom Logging Filter (20 min)

3.1 Global Filter to Log Requests

package com.example.gateway.filter;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.cloud.gateway.filter.GatewayFilterChain;
import org.springframework.cloud.gateway.filter.GlobalFilter;
import org.springframework.core.Ordered;
import org.springframework.http.server.reactive.ServerHttpRequest;
import org.springframework.stereotype.Component;
import org.springframework.web.server.ServerWebExchange;
import reactor.core.publisher.Mono;

import java.util.UUID;

@Component
public class LoggingFilter implements GlobalFilter, Ordered {
    
    private static final Logger log = LoggerFactory.getLogger(LoggingFilter.class);
    
    @Override
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        // Generate a correlation ID
        String correlationId = UUID.randomUUID().toString().substring(0, 8);
        long startTime = System.currentTimeMillis();
        
        ServerHttpRequest request = exchange.getRequest();
        
        // Log the incoming request
        log.info(">>> [{}] Incoming: {} {} from {}", 
            correlationId,
            request.getMethod(),
            request.getURI().getPath(),
            request.getRemoteAddress()
        );
        
        // Add the correlation ID to the header
        ServerHttpRequest mutatedRequest = request.mutate()
            .header("X-Correlation-Id", correlationId)
            .build();
        
        ServerWebExchange mutatedExchange = exchange.mutate()
            .request(mutatedRequest)
            .build();
        
        return chain.filter(mutatedExchange)
            .then(Mono.fromRunnable(() -> {
                long duration = System.currentTimeMillis() - startTime;
                log.info("<<< [{}] Response: {} | Duration: {}ms", 
                    correlationId,
                    exchange.getResponse().getStatusCode(),
                    duration
                );
            }));
    }
    
    @Override
    public int getOrder() {
        return -100; // Executed very early
    }
}

3.2 Filter to Add Response Headers

package com.example.gateway.filter;

import org.springframework.cloud.gateway.filter.GatewayFilterChain;
import org.springframework.cloud.gateway.filter.GlobalFilter;
import org.springframework.core.Ordered;
import org.springframework.http.server.reactive.ServerHttpResponse;
import org.springframework.stereotype.Component;
import org.springframework.web.server.ServerWebExchange;
import reactor.core.publisher.Mono;

@Component
public class ResponseHeaderFilter implements GlobalFilter, Ordered {
    
    @Override
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        return chain.filter(exchange)
            .then(Mono.fromRunnable(() -> {
                ServerHttpResponse response = exchange.getResponse();
                response.getHeaders().add("X-Gateway-Version", "1.0.0");
                response.getHeaders().add("X-Powered-By", "Spring Cloud Gateway");
            }));
    }
    
    @Override
    public int getOrder() {
        return -1;
    }
}

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Part 4: CORS Configuration (10 min)

4.1 CORS Configuration

package com.example.gateway.config;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.cors.CorsConfiguration;
import org.springframework.web.cors.reactive.CorsWebFilter;
import org.springframework.web.cors.reactive.UrlBasedCorsConfigurationSource;

import java.util.Arrays;
import java.util.List;

@Configuration
public class CorsConfig {
    
    @Bean
    public CorsWebFilter corsWebFilter() {
        CorsConfiguration corsConfig = new CorsConfiguration();
        
        // Allowed origins
        corsConfig.setAllowedOrigins(List.of(
            "http://localhost:3000",    // React dev
            "http://localhost:4200",    // Angular dev
            "http://localhost:8080"     // Same origin
        ));
        
        // Allowed methods
        corsConfig.setAllowedMethods(Arrays.asList(
            "GET", "POST", "PUT", "DELETE", "OPTIONS", "PATCH"
        ));
        
        // Allowed headers
        corsConfig.setAllowedHeaders(List.of("*"));
        
        // Credentials
        corsConfig.setAllowCredentials(true);
        
        // Cache preflight
        corsConfig.setMaxAge(3600L);
        
        UrlBasedCorsConfigurationSource source = new UrlBasedCorsConfigurationSource();
        source.registerCorsConfiguration("/**", corsConfig);
        
        return new CorsWebFilter(source);
    }
}

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Part 5: Test the Gateway (15 min)

5.1 Startup Order

1. Eureka Server (8761)
2. User Service (8081)
3. Product Service (8083)
4. API Gateway (8080)

# Terminal 1
cd eureka-server && mvn spring-boot:run

# Terminal 2
cd user-service && mvn spring-boot:run

# Terminal 3
cd product-service && mvn spring-boot:run

# Terminal 4
cd api-gateway && mvn spring-boot:run

5.2 Test the Routes

# Via Gateway - Users
curl http://localhost:8080/api/users
curl http://localhost:8080/api/users/1

# Via Gateway - Products
curl http://localhost:8080/api/products
curl http://localhost:8080/api/products/1

# Check response headers
curl -i http://localhost:8080/api/users
# Should include:
# X-Gateway-Version: 1.0.0
# X-Powered-By: Spring Cloud Gateway

5.3 Test the Fallback

# Stop User Service (Ctrl+C in its terminal)

# Wait 30 seconds (Eureka eviction)

# Test via Gateway
curl http://localhost:8080/api/users

# Expected result:
{
  "status": "SERVICE_UNAVAILABLE",
  "message": "User Service is currently unavailable. Please try again later.",
  "timestamp": "2024-01-15T10:30:00",
  "service": "user-service"
}

5.4 View Active Routes

curl http://localhost:8080/actuator/gateway/routes

Response:

[
  {
    "predicate": "Paths: [/api/users/**], match trailing slash: true",
    "route_id": "user-service",
    "filters": ["CircuitBreaker name='userServiceCB' fallback='forward:/fallback/users'"],
    "uri": "lb://user-service",
    "order": 0
  },
  {
    "predicate": "Paths: [/api/products/**], match trailing slash: true",
    "route_id": "product-service",
    "filters": ["CircuitBreaker name='productServiceCB' fallback='forward:/fallback/products'"],
    "uri": "lb://product-service",
    "order": 0
  }
]

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✅ Validation Checklist

• API Gateway starts on http://localhost:8080
• curl http://localhost:8080/api/users → response from User Service
• curl http://localhost:8080/api/products → response from Product Service
• Headers X-Gateway-Version and X-Powered-By present
• Request logs with correlation ID
• Fallback works when a service is down
• /actuator/gateway/routes lists the routes

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🎯 Bonus Exercises

Exercise 1: Rate Limiting

Add a rate limiter (requires Redis):

filters:
  - name: RequestRateLimiter
    args:
      redis-rate-limiter.replenishRate: 10
      redis-rate-limiter.burstCapacity: 20

Exercise 2: Rewrite Path

Redirect /v1/users/** to /api/users/**:

- id: user-service-v1
  uri: lb://user-service
  predicates:
    - Path=/v1/users/**
  filters:
    - RewritePath=/v1/(?<segment>.*), /api/$\{segment}

Exercise 3: Simple Authentication

Create a filter that checks for the presence of an X-API-Key header.

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TP13.4: Feign + Resilience4j →

Back to Module 13